Device for measuring the anchoring forces in prestressed ground or rock anchors

ABSTRACT

The device for measuring the anchoring forces in prestressed ground or rock anchors has a cushion disposed between the anchoring device and an abutment plate to which the anchoring force is transmitted which is charged with pressure agent, and a pressure head on the abutment plate has mounted on it a bolt for operating an electric switch that operates an indicator that reflects the start of movement.

United States Patent Finsterwalder July 24, 1973 DEVICE FOR MEASURINGTHE [56] References Cited ANCHORING FORCES IN PRESTRESSED UNITED STATESPATENTS GROUND 0R ROCK ANCHORS 3,111,655 ll/l963 Kolarsky et a1 85/62 x[75] Inventor: Ktlemcsans Finstervalder, Sucking FOREIGN PATENTS ORAPPLICATIONS many 991,783 5/1965 Great Britain 85/62 [73] Assignee:Firma Dyckerhofi & Wldmann Aktiengesellschait, Socking uber PrimaryExaminer-Richard C. Queisser Starnberg, Germany AssistantExaminer-Stephen A. Kreitman [22] Filed: June 30, 1971 Attorney-RobertH. Jacob [21] Appl. No.: 158,382 [57] ABSTRACT The device for measuringthe anchoring forces in pre- [30] Foreign Appncation p i it Datastressed ground or rock anchors has a cushion disposed between theanchoring device and an abutment plate to July 25, 1970 Germany P 20 37060.9 which the anchoring force is transmitted which is 7 charged withpressure agent, and a pressure head on 'g 8 1 562 the abutment plate hasmounted on it a boltfor operat- [58] Fie'ld 73788 F 88 ing an electricswitch that operates an indicator that refleets the start of movement.

5 Claims, 2 Drawing Figures PATENTED 3,747. 400

INVEN TOR. Klemens F/NS TERWA LDER DEVICE FOR MEASURING THE ANCHORINGFORCES IN PRESTRESSED GROUND OR ROCK ANCHORS BACKGROUND OF THE INVENTIONThe invention relates to a device for measuring the anchoring forcesabsorbed in prestressed or rock anchors by way of anchoring devices,such as, for example, anchoring plates.

In order to make it possible to examine the effectiveness of ground androck anchors, measuring means or devices are necessary which over aperiod of years can reliably measure the forces that are present in theanchor. A particular problem in this connection is to so construct thesemeasuring devices that they are nonsensitive against temperature,moisture, soiling and corrosion and that, above all, the measuredquantity of the zero reading to which the device has been set does notchange.

The simplest manner of measuring the forces in a prestressed anchorconsists in mounting a hydraulic tension press such as is utilized forthe tensioning of tensioning elements in the prestressed concreteconstruction. By introducing a pressure agent in the pressure space ofthe press, balance with the force to be measured is then established.The pressure in the hydraulic system of the press that is being measuredat any moment in which the anchoring body, i.e., for example, a threadednut, can be loosened from the anchoring device, is then a measure forthe force or strength in the anchor. This method, however, is cumbersomeand expensive, since a tensioning press mustbe set up for eachmeasurement because it is difficult to reliably determine the point oftime at which balance is established, i.e., when the force to bemeasured is transferred from the anchoring device to the hydraulicsystem.

Other measuring devices operate with pressure springs between theanchoring body and the anchoring device; in this connection themagnitude of the compression of these springs is a measure for theexisting anchoring forces. Finally also the forces present in the anchorcan be determined by means of the frequency of a tensionedoscillatingmusic wire. All of these systems, however, have theshortcoming that the forces are determined indirectly, i.e., by way ofdeforming or frequency measurement, and that the initial value (zeroreading) of the measurement has to be kept constant for the duration ofthe measurement, i.e., as the case may be, overa period of years.

It is an object of the invention to provide a measuring device which, byvirtue of 'its construction and function, provides the prerequisite foran invariable zero reading and simultaneously affords a possibility ofcontrolling the anchor strength at small expense at any time desired.

SUMMARY OF THE INVENTION This problem is solved by the invention in thatbetween the anchoring device and a further abutment or counter bearingbody, for example an anchoring plate onto which the anchoring force isdirectly transmitted by the anchoring head, an enclosed or cased presswhich lies flat in condition of rest, that can be charged or loaded witha hydraulic pressure agent, is arranged, and that a measuring device isprovided for determining the start of the movement of the counterbearing body when the enclosed press is charged or loaded. For thispurpose an electric switch, for example a microswitch, may be providedthat is coupled with an indicating device.

In order to constitute the device in accordance with the invention as apermanent surveillance and control system, the pressure duct to theenclosed press may be closed and a pressure container filled with gasconnected thereto as resilient element.

The advantage of the invention will be found in the first place in thatfor the actual measurement of the anchoring force a hydraulic pressuremeans is employed, which permits in a manner known per se, by way of thepressure in the system and the magnitude of the loaded surface, a simpledetermination of the anchoring force. By the use of a pressure cushion,which in the simplest case consists of two annular pieces of sheet metalwhich rest flatly against one another and are welded together alongtheir inner and outer circumferences, it is insured that the anchoringforce is transferred by flat contact from metal to metal without anyelements being interposed therebetween which could change the startingvalue of the measurement in any manner. For the control of the anchoringforce it is merely necessary to apply pressure to the pressure cushion,which is possible without difficulty due to its small hollow space, andexactly catch the start of the movement of the anchor head, i.e., of theouter part of the anchor device separated by the pressure cushion. Forthis purpose an electric microswitch is preferably provided which iscapable of closing its circuit already after a movement of 0.15mm.

BRIEF DESCRIPTION OF THE DRAWINGS The invention is explained more indetail hereinafter with reference to an embodiment illustrated in theaccompanying drawings, in which FIG. I is a cross-section through theanchoring head, and

FIG. 2 is a cross-section through the anchoring body and the counterbearing member with pressure cushion, drawn to an enlarged scale.

DESCRIPTION OF THE INVENTION In the embodiment of the inventionillustrated in FIG. 1 a rock anchor l is anchored on the outer surfaceof a building site 3 that is secured by means of sprayed concrete 2. Theactual anchorage is disposed in a concrete socket 4.

The anchorage comprises an anchoring plate 5 which rests against theconcrete socket 4 and a further anchoring or outer plate 6 against whichthe anchoring nut 7 is'supported. The anchoring nut 7 is secured uponthe surface of the anchoring head 8 by means of a thread. The anchorproper may, as shown by the example, consist of a number of individualelements 9 which are separately anchored with respect to the anchoringhead 8.

Between the two anchoring plates 5 and 6 a small pressure cushion 10 islocated. The pressure cushion 10 comprises two pieces of sheet metal ofannular configuration that are in direct engagement with one another,which are tightly connected with one another at the inner and outerborders 11 and 12, and into the hollow space of which leads supply tube13. The supply tube 13 extends through a suitable bore 14 in the outeranchor plate 6. Through the supply tube 13 the pressure cushion can bechanged with a pressure agent, for example hydraulic oil.

In the condition of rest, i.e., in the condition of the anchor undertension, the anchoring force is transferred by way of the anchoring nut7 to the outer anchor plate 6 and from this to the lower anchor plate byway of the plates of the pressure cushion 10 that rest closely againsteach other with the anchoring force being transferred from the anchoringplate to the concrete socket 4. In this condition a direct take-off ofthe anchor force is assured without it being possible that fatiguephenomena would appear during the course of time.

If the anchoring force is to be measured, then pressure medium ispressed into the pressure cushion by way of the supply duct 13 until thepressure in the cushion is in balance with the prestressing force of theanchor. This point of time is recognized in that the outer anchor plate6 rises from the inner anchor plate 5. In order to make it possible toquickly and safely determine this operation of movement, an electricmicroswitch is provided in the embodiment illustrated in the drawings. I

For this purpose a strap 15 is secured to the lower anchor plate 5 bymeans of screws 16. At the upper bridge 17 of the stirrup 15 a bolt 18that is adjustable by means of a fine thread is arranged which can bebrought into contact with the microswitch 19 which is secured to theouter anchor plate or, as illustrated in FIG. 2, directly on the anchorhead 8 by means ofa stirrup 20 (FIGfZ).

With suitable setting of the switch contact the start of the movement ofthe anchor head 8 can be exactly determined. A signal transmitter can beactuated by way of the electric microswitch 19, for example a buzzer ora control light, that exactly signals the point of time of the start ofthe movement to the operating personnel when the manometer at thepressure pump has to be read. From the surface of the pressure cushionand the oil pressure that can be readily read on a manometer, which isnot illustrated, the prestressing force of the anchor can be determined.

Under these conditions it is not important for the measurement to catchthe beginning of the movement exactly. Minor inaccuracies introducemerely negligible deviations. The reason for that is found in that afterthe'prestressing force is reached, the pressure in the pressure cushionrises only very slightly. In this connection the magnitude of the risein pressure depends on the path of movement of the anchor plate and onthe structural features of the anchor. lts influence on the measurementof the anchoring force thus can be readily taken into consideration. Itmust be observed that this manner of measurement can be undertaken atany desired point of time after the installation of the anchor and asoften as desired.

If a permanent visible control of the-available anchoring force isrequired, then the measuring device can be supplemented in a simplemanner. For this purpose merely the pressure duct 13 to the pressurecushion 10 is closed after that pressure is reached which is requiredand sufficient for raising the anchor, and a bottle filled with gas isconnected to the pressure duct as resilient element. The measurementthen is effected as follows:

First the pressure in the system is increased until the anchor plate 6has been raised about 0.25mm from the anchor plate 5. Then the pressureduct is closed. By this operation the gas in the pressure flask isstrongly compressed. Then it has an effect similar to that of a verysoft spring in relation to the pressure system. Small leakage losses inthe conduit system therefore result only in very small, hardlynoticeable, losses of pressure in the total system, so that one canstart out with sufficient accuracy from a constant pressure P. If theforce in the anchor changes, movements again take place in the anchorhead 8 or the anchor plate 6 which can be registered by means of ametering clock or microswitches. In order to make it possible to takereadings from both directions, it is of course necessary to provideeffective switches in both directions.

This arrangement affords the further advantage that the ability of themeasuring device to function can be tested at any time without it beingnecessary to release the load from the anchor.

Having now described my invention with reference to the embodimentillustrated, what I desire to protect by letters patent is set forth inthe appended claims.

I claim:

1. Device for indicating a change in the anchoring forces absorbed byway of anchoring means such as anchor plates in prestressed ground orrock anchors comprising an anchoring element such as a plate, an outerplate defining an abutment member disposed proximate said element, anannular pressure cushion that is flat in the condition of rest, disposedintermediate said element and said outer plate and an anchoring headhaving a nut proximate said outer plate in position to transfer forcesdirectly to said outer plate, means for applying pressure to saidcushion, and a measuring device arranged between said anchoring elementand said outer plate for determining the start of the movement of saidouter plate as pressure is applied to said cushion.

2. Device in accordance with claim 1, where said measuring deviceincludes an electric switch adapted to be connected to an indicatingdevice.

3. Device in accordance with claim 2 including a stirrup mounted on saidanchoring head, a strap connected to said anchoring element, and a boltextending between said stirrup and said switch.

4. Device in accordance with claim 3 including adjusting means for saidbolt on said strap.

5. Device in accordance with claim 1, where said means for applyingpressure includes a closed duct in communication with said cushion andthat said duct is adapted to be connected to a container filled with apressure agent.

1. Device for indicating a change in the anchoring forces absorbed by way of anchoring means such as anchor plates in prestressed ground or rock anchors comPrising an anchoring element such as a plate, an outer plate defining an abutment member disposed proximate said element, an annular pressure cushion that is flat in the condition of rest, disposed intermediate said element and said outer plate and an anchoring head having a nut proximate said outer plate in position to transfer forces directly to said outer plate, means for applying pressure to said cushion, and a measuring device arranged between said anchoring element and said outer plate for determining the start of the movement of said outer plate as pressure is applied to said cushion.
 2. Device in accordance with claim 1, where said measuring device includes an electric switch adapted to be connected to an indicating device.
 3. Device in accordance with claim 2 including a stirrup mounted on said anchoring head, a strap connected to said anchoring element, and a bolt extending between said stirrup and said switch.
 4. Device in accordance with claim 3 including adjusting means for said bolt on said strap.
 5. Device in accordance with claim 1, where said means for applying pressure includes a closed duct in communication with said cushion and that said duct is adapted to be connected to a container filled with a pressure agent. 